1. Field of the Invention
This invention relates to thermoplastic condensation polymers which are terpolymers having aliphatic polyester, polysiloxane and polycarbonate segments (blocks). These polymers exhibit advantageous melt flows, advantageous low temperature properties and resistance to solvents, chemicals, hydrolysis and to photodecomposition. The terpolymers are especially useful as engineering thermoplastics.
2. Brief Description of the Related Art
Condensation copolymers having polysiloxane and polycarbonate blocks are known. Representative of such polymers are those disclosed by Schmidt et al., U. S. Pat. No. 4,681,922 (July 1987), Vaughn, U.S. Pat. No. 3,189,662 (June 1965), Vaughn, U.S. Pat. No. 3,419,635 (Dec. 1968), and Merritt, U.S. Pat. No. 3,832,419 (Aug. 1974). Some of these copolymers, while useful, have slower than desired flow properties, requiring high torque or high molding pressures during processing. From the standpoint of ease of processing, it is desirable for a thermoplastic to have higher melt flow properties. This makes possible rapid and complete mold filling and is especially important for molding complex and thin-walled articles.
Other siloxane-carbonate copolymers, such as described by Vaughn, U.S. Pat. No. 3,419,635, have an elastomeric character and are not considered as engineering thermoplastics, being more useful as adhesives, coatings, sealants, roofing material, impact modifying additives and the like.
A shortcoming of other siloxane-carbonate polymers is inadequate impact strength at low temperatures.
Another property which needs improvement beyond the levels achieved with the prior art copolymers is solvent resistance, as manifested for instance by resistance to crazing upon exposure to solvents, motor fuels, and the like.
A shortcoming of certain other known polycarbonate-siloxane copolymers is the presence of an aryloxysilicon linkage, which is hydrolysisprone. Some other known polycarbonate-siloxane copolymers have aromatic acyloxyphenoxy linkages which are photolabile, causing discoloration on exposure to sunlight or fluorescent light (believed to be caused by the known photo-Fries rearrangement reaction).
The thermoplastics of the present invention exhibit advantageous low temperature impact strengths, melt flow solvent resistance, hydrolytic stablility and photo-stability.
An advantageous feature of the invention from a process standpoint is the formation of the block polymer in a convenient and novel one-step process which forms the aliphatic polyester and carbonate block and links it with the polysiloxane block. This is in contrast to processes for making block copolymers, where it is usually necessary to synthesize the individual blocks and to combine them in a separate step, thus imposing additional labor and time on the process.